Bugs can be reported to developers via https://bugzilla.mozilla.org, a system which also will publish your email address and make it available to spammers.
Below there are some possible bugs that users where unwilling to reported via bugzilla.
alert ( Date.UTC(2006, 5, 31) - Date.UTC(2006, 6, 01)); FF126.96.36.199 returns 0;
This bug prevents FireFox from keeping you logged in to websites. Each time you open and close Firefox, you will have to re-login to sites.